Broadband amplifiers and power amplifiers, in particular, cause distortion to a signal, since the amplifiers function non-linearly at least to some degree. Besides distortion amplifiers cause for example noise. The influence of distortion caused by amplifiers has been reduced by means of different connections. In feedforward amplifiers distortion has been reduced by separating distortion from a signal after amplification. After separation, distortion has been added as negative distortion to a signal containing distortion, and thus the distortion components have canceled each other out. The resulting amplified signal has contained considerably less distortion than an unamplified signal.
Usually, the incoming end and outgoing end of the amplifier have to be adapted to signal paths. Processing of signals always causes some delay to a signal. For this reason adaptation of the delay and gain requires particular accuracy so that the amplifier would function as desired in all operational environments. Prior art amplifiers have been provided with signal cancellation loops and error cancellation loops, where the phase and amplitude of the influencing signal have been adjusted. Adjustment has enabled reduction of distortion in the signal. The amplifier has been part of a control arrangement which has been able to adapt to the received signal to some degree.
Prior art amplifiers cannot, however, adapt to a signal if the power level of the incoming signal varies a lot, which is the case e.g. in CDMA radio systems. The temperature also causes problems to adaptation. The above-mentioned issues render signal adaptation more difficult since the non-linearities of an amplifier depend greatly on the fluctuation of the power level and on the temperature.
Special factors, which can be denoted with .alpha. and .beta., are employed for adjusting the phase and amplitude. The factors are changed and adjusted by means of an adaptation factor, which is denoted with K. When the power level of an input signal changes rapidly, the adaptation factor also needs to change rapidly to correspond to the changed situation. In prior art solutions adaptation factors have, however, been constant, and thus the adaptation of the input signal has occurred too slowly. Slow adaptation has resulted in inaccurate adjustment.